Title: Centrality and multiparticle production in ultrarelativistic nuclear collisions

Abstract

A critical analysis of methods for selecting central events in high-energy proton–nucleus (pA) and nucleus–nucleus (AA) collisions is presented. A sample of event classes in which background fluctuations associated with the dispersion of the impact parameter of each event or the number of participant nucleons are minimal is examined. At the SPS and LHC energies, the numbers of nucleon–nucleon collisions are estimated with the aid of the Monte Carlo event generators HIJING and AMPT, which take into account energy–momentum conservation, and on the basis of a non-Glauber model involving string fusion and a modified Glauber model. The results obtained in this way demonstrate the need for revising the extensively used application of the Glauber model in normalizing multiplicity yields in experimental data on pA and AA collisions in the soft region of the spectrum.

@article{osti_22612616,
title = {Centrality and multiparticle production in ultrarelativistic nuclear collisions},
author = {Drozhzhova, T. A. and Kovalenko, V. N. and Seryakov, A. Yu. and Feofilov, G. A., E-mail: grigory-feofilov@yandex.ru},
abstractNote = {A critical analysis of methods for selecting central events in high-energy proton–nucleus (pA) and nucleus–nucleus (AA) collisions is presented. A sample of event classes in which background fluctuations associated with the dispersion of the impact parameter of each event or the number of participant nucleons are minimal is examined. At the SPS and LHC energies, the numbers of nucleon–nucleon collisions are estimated with the aid of the Monte Carlo event generators HIJING and AMPT, which take into account energy–momentum conservation, and on the basis of a non-Glauber model involving string fusion and a modified Glauber model. The results obtained in this way demonstrate the need for revising the extensively used application of the Glauber model in normalizing multiplicity yields in experimental data on pA and AA collisions in the soft region of the spectrum.},
doi = {10.1134/S1063778816040074},
journal = {Physics of Atomic Nuclei},
number = 5,
volume = 79,
place = {United States},
year = 2016,
month = 9
}

Inclusive double differential multiplicities [ital d][sup 2][ital N]/[ital dy] d[ital p][sub [ital t]] and related quantities have been measured for protons and deuterons produced in 14.6[ital A] GeV/[ital c] Si+Al and Si+Pb collisions using the E814 forward spectrometer at the AGS at BNL. Collision centrality'' is determined by measuring [ital N][sub [ital c]], the total charged particle multiplicity in the pseudorapidity range 0.85[lt][eta][lt]3.8. For both systems Si + Al and Si + Pb, an increase in the proton rapidity distribution [ital dN]/[ital dy] at midrapidity and a corresponding decrease at higher rapidities are observed with increasing [ital N][sub [ital c]].more » For Si+Pb, Boltzmann slope parameters [ital T][sub [ital B]] increase significantly in the most central collisions. The measured distributions exhibit a centrality dependence even when [sigma]/[sigma][sub geo][approx lt]10%, where full overlap between the Si and Pb nuclei occurs in a simple geometric picture. The proton rapidity distribution [ital dN]/[ital dy] is presented for the symmetric system Si+Al over the entire rapididty interval. The total number of protons, which is the integral of this quantity over rapidity, varies with [ital N][sub [ital c]]. Results are compared with various model calculations, mostly using the hadronic cascade codes ARC and RQMD. No significant nuclear transparency is observed, indicating that large baryon and energy densities are produced in these collisions.« less

In ultrarelativistic heavy ion collisions, the multiplicity of multistrange baryons per participating nucleon increases with centrality in a different fashion for different systems and energies. At RHIC, for copper+copper (CuCu) collisions the increase is much steeper than for gold-gold (AuAu) collisions. We show that this system size dependence is due to a core-corona effect: the relative importance of the corona as compared to the core (thermalized matter) contribution varies and the contribution of a corona nucleon to the multiplicity differs from that of a core nucleon. {phi} mesons follow--as all hadrons--the same trend, but the difference between core and coronamore » multiplicity is relatively small, and therefore the CuCu and AuAu results are quite similar. This simple geometrical explanation makes also a strong case in favor of the validity of Glauber geometry in the peripheral regions of ultrarelativistic heavy ion collisions, which is crucial for understanding the early evolution of the system.« less

We have carried out a wide study of Cold Nuclear Matter (CNM) effects on J/{Psi} = production in dAu, CuCu and AuAu collisions at {radical}s{sub NN} = 200 GeV. We have studied the effects of three different gluon-shadowing parameterizations, using the usual simplified kinematics for which the momentum of the gluon recoiling against the J/{Psi} is neglected as well as an exact kinematics for a 2 {yields} 2 process, namely g + g {yields} J/{psi} + g as expected from LO pQCD. We have shown that the rapidity distribution of the nuclear modification factor R{sub dAu}, and particularly its anti-shadowingmore » peak, is systematically shifted toward larger rapidities in the 2 {yields} 2 kinematics, irrespective of which shadowing parameterization is used. In turn, we have noted differences in the effective final-state nuclear absorption needed to fit the PHENIX dAu data. Taking advantage of our implementation of a 2 {yields} 2 kinematics, we have also computed the transverse momentum dependence of the nuclear modification factor, which cannot be predicted with the usual simplified kinematics. All the corresponding observables have been computed for CuCu and AuAu collisions and compared to the PHENIX and STAR data. Finally, we have extracted the effective nuclear absorption from the recent measurements of RCP in dAu collisions by the PHENIX collaboration.« less

We have carried out a wide study of cold nuclear matter (CNM) effects on J/psi production in dAu, CuCu and AuAu collisions at sq root(s{sub NN})=200 GeV. We have studied the effects of three different gluon-shadowing parametrizations, using the usual simplified kinematics for which the momentum of the gluon recoiling against the J/psi is neglected as well as an exact kinematics for a 2->2 process; namely g+g->J/psi+g as expected from LO pQCD. We have shown that the rapidity distribution of the nuclear modification factor R{sub dAu}, and particularly its antishadowing peak, is systematically shifted toward larger rapidities in the 2->2more » kinematics, irrespective of which shadowing parametrization is used. In turn, we have noted differences in the effective final-state nuclear absorption needed to fit the PHENIX dAu data. Taking advantage of our implementation of 2->2 kinematics, we have also computed the transverse momentum dependence of the nuclear modification factor, which cannot be predicted with the usual simplified kinematics. All the corresponding observables have been computed for CuCu and AuAu collisions and compared to the PHENIX and STAR data. Finally, we have extracted the effective nuclear absorption from the recent measurements of R{sub CP} in dAu collisions by the PHENIX collaboration.« less